This invention relates generally to nitrogen oxide (NOx) emissions produced by internal combustion engines in a vehicle and more particularly to a system for controlling reduction of the NOx emissions by means of a selective catalytic reduction (SCR) method.
The invention is particularly applicable to and will be described with specific reference to a control system for regulating the supply of an external reductant, ammonia, to a reducing catalyst in an SCR system taking into account the effect of NOx transient emissions produced in vehicles powered by diesel engines. However, those skilled in the art will recognize that the control system has broader applications and may be applied to SCR systems using other reductants such as fuel oil or hydrocarbons as well as SCR systems used in other mobile internal combustion engine applications, such as gasoline engines employing xe2x80x9clean burnxe2x80x9d techniques.
The following patents and publications are incorporated by reference herein and made a part hereof:
1) U.S. Pat. No. 4,403,473, to John R. Gladden, dated Sep. 13, 1983, entitled: xe2x80x9cAMMONIA/FUEL RATIO CONTROL SYSTEM FOR REDUCING NITROGEN OXIDE EMISSIONSxe2x80x9d;
2) SAE Paper No. 952493, by H. Luders, R. Backes, and G. Huthwohl, FEV Motoremtechnik and D. A. Ketcher, R. W. Horrocks, R. G. Hurley and R. H. Hammerle, Ford Motor Co., dated Oct. 16-19, 1995, entitled: xe2x80x9cAN UREA LEAN NOx CATALYST SYSTEM FOR LIGHT DUTY DIESEL VEHICLESxe2x80x9d (See page 7);
3) SAE Paper No. 921673, by J. Walker, Ortech and B. K. Speronello, Engelhard Corp., dated Sep. 14-17, 1992, entitled: xe2x80x9cDEVELOPMENT OF AN AMMONIA/SCR NOx REDUCTION SYSTEM FOR A HEAVY DUTY NATURAL GAS ENGINExe2x80x9d;
4) U.S. Pat. No. 5,606,855, to Naoki Tomisawa, dated Mar. 4, 1997, entitled: xe2x80x9cAPPARATUS AND METHOD FOR ESTIMATING THE TEMPERATURE OF AN AUTOMOTIVE CATALYTIC CONVERTERxe2x80x9d; and,
5) U.S. Pat. No. 5,490,064, to Minowa et al., dated Feb. 6, 1996, entitled: xe2x80x9cCONTROL UNIT FOR VEHICLE AND TOTAL CONTROL SYSTEM THEREFORxe2x80x9d.
None of the material cited above form any part of the present invention. The material is incorporated by reference herein so that details relating to SCR systems such as the operation of the SCR systems with ammonia or hydrocarbon reductants, the metering of the reductants, principles and control of engine operation etc., need not be set forth or described in detail herein.
This invention is directed to the removal of nitrogen oxides (NOx) from the exhaust gases of internal combustion engine, particularly diesel engines, which operate at combustion conditions with air in large excess of that required for stoichiometric combustion, i.e., lean. Unfortunately, the presence of excess air makes the catalytic reduction of nitrogen oxides difficult. Emission regulations impose a limit on the quantity of specific emissions, including NOx, that a vehicle can emit during a specified drive cycle, such as i) for light duty trucks, an FTP (xe2x80x9cfederal test procedurexe2x80x9d) in the United States or an MVEG (xe2x80x9cmobile vehicle emissions groupxe2x80x9d) in Europe or ii) for heavy duty trucks, a Heavy Duty Cycle in the United States or an ESC (European Steady State Cycle) or ETC (European Transient Cycle) in Europe. The regulations are increasingly limiting the amount of nitrogen oxides that can be emitted during the regulated drive cycle.
There are numerous ways known in the art to remove NOx from a waste gas. This invention is directed to a catalytic reduction method for removing NOx. A catalytic reduction method essentially comprises passing the exhaust gas over a catalyst bed in the presence of a reducing gas to convert the NOx into nitrogen. Conventionally, there are three ways to treat vehicular exhaust to reduce NOx. The first method is non-selective catalyst reduction (NSCR). The second way is selective non-catalytic reduction (SNCR) and the last method is selective catalyst reduction (SCR). This invention relates to SCR systems.
In diesel engines, sufficient NOx reduction to meet current regulations has been achieved by combustion modifications in the diesel engine by, for example, incorporating EGR. Projected emission levels are such that combustion and engine modifications will not be sufficient to meet the more stringent levels. Because of excess oxygen present in diesel exhaust gases, the opportunity for NOx reduction under rich or stoichiometric air/fuel is not possible. SCR is a technology that has been shown effective in removing NOx from oxygen rich exhaust. A number of SCR systems have been developed which, because of infrastructure concerns, have used diesel fuel or diesel oil as the reductant source. Unfortunately, as of this date, an HC reducing catalyst has not yet been developed which has sufficient activity and is effective over the entire operating range of the diesel engine.
A common nitrogen oxide reducing agent, long used in industrial processes, is ammonia. NOx reducing catalysts have been developed which are effective over the operating range of the engine. Despite the infrastructure concerns relating to the use of urea in a mobile application as well as the potentially dangerous risks of ammonia break-through or slip, ammonia SCR systems are becoming the favored choice for mobile applications to meet the more stringent NOx emissions. This is, among other reasons, because of the high NOx conversion percentages possible with ammonia coupled with the ability to optimize the combustion process for maximum power output with minimum fuel consumption.
Notwithstanding what may be said to be inherent advantages of an ammonia based SCR system, the control systems to date have been excessively complicated and/or ineffective to control the SCR system when the impact of NOx transient emissions on the SCR system is considered. As will be shown below, if the transient NOx emissions can not be adequately reduced by the SCR control system, then stringent NOx emission regulations will not be met.
Early patents controlled ammonia metering by considering the emissions to be controlled at steady state conditions. For example, U.S. Pat. No. 4,403,473 to Gladden (Sep. 13, 1983) considered NOx emissions at various speed ranges and concluded that a linear relationship exists between fuel flow and NOx. (Earlier Gladden U.S. Pat. No. 4,188,364, Feb. 12, 1980 concluded that ammonia catalyst adsorbed ammonia at temperatures lower than 200xc2x0 C. and desorbed at temperatures between 200-800xc2x0 C., the SCR system should operate at higher temperatures to achieve complete reaction between ammonia and NOx.) Thus, in Gladden ""473, the fuel mass flow is sensed and NH3 throttled at a percentage of fuel flow provided the temperature of the gases in the catalytic converter are within a set range. This basic control concept is used today in most mobile, ammonia SCR systems. For example, U.S. Pat. No. 5,116,579 to Kobayashi et al. (May 26, 1992) additionally measures the humidity of intake air and one or more operating parameters of engine power, intake air temperature, fuel consumption and exhaust gas temperature to set an ammonia ratio control valve. The molar ratio of ammonia to NOx is set at less than one (sub-stoichiometric) to minimize ammonia slip.
Typically the reductant is pulse metered into the exhaust gas stream in a manner similar to that used for operating conventional fuel injectors. In U.S. Pat. No. 4,963,332 to Brand et al. (Oct. 16, 1990), NOx upstream and downstream of the catalytic converter is sensed and a pulsed dosing valve controlled by the upstream and downstream signals. In U.S. Pat. No. 5,522,218 to Lane et al. (Jun. 4, 1996), the pulse width of the reductant injector is controlled from maps of exhaust gas temperature and engine operating conditions such as engine rpm, transmission gear and engine speed.
As noted, the industrial art has long used ammonia in SCR systems to control NOx emissions typically by set point control loops such as shown in U.S. Pat. No. 5,047,220 to Polcer (Sep. 10, 1991) in which a downstream NOx sensor is used to generate a trim signal in the control loop. The industrial art has also recognized that changes in load from the turbine, furnace etc. affects the ammonia SCR systems. Thus in U.S. Pat. No. 4,314,345 to Shiraishi et al. (Feb. 2, 1982), variations in load are determined by sensing the temperature of the exhaust gas. When the exhaust gases are at certain temperature ranges a variation in the load is assumed to occur and different or predicted NH3/NOx molar ratios are used to account for the adsorption/desorption characteristics of the catalyst. A more sophisticated molar ratio control system is disclosed in U.S. Pat. No. 4,751,054 to Watanabe. Watanabe uses not only upstream and downstream NOx sensors but also temperature, flow rate and NH3 detectors to set a mole ratio correcting signal. In U.S. Pat. No. 4,473,536 to Carberg et al. (Sep. 25, 1984) a turbine""s inlet airflow, discharge pressure, discharge temperature and mass fuel flow are sensed to predict NOx generated by the turbine which signal is corrected for NOx error by time delayed NOx sensor measurements. Carberg recognizes that turbine load changes may change NOx emissions in a time frame quicker than the 1 plus minute needed to determine the NOx emissions in a gas sample with conventional NOx sensors and thus makes a prediction, which can not be corrected in real time. The industrial systems, for the most part, do not operate under the highly transient conditions which characterize vehicle engines producing sudden NOx transients. Industrial systems also operate in an environment in which samples of the gas being produced can be taken to accurately determine the NOx content to trim the ammonia metering valve in closed loop control.
In addition to systems which sense engine operating parameters to control metering of ammonia or a reductant, there are other approaches used to control NOx emissions in mobile applications. In U.S. Pat. No. 5,845,487 to Fraenkle et al. (Dec. 8, 1998), the exhaust gas temperature is sensed. If the exhaust gas is outside the temperature limits at which the SCR system is effective i.e., below the operating temperature, the fuel injection timing to the engine is retarded, reducing the NOx via combustion modifications. In U.S. Pat. No. 5,842,341 to Kibe (Dec. 1, 1998) space velocity and exhaust gas temperature is measured to determine the reductant quantity. In addition inlet and outlet catalytic converter temperature is measured and reductant flow is decreased from the steady state conditions when the temperature differential between inlet and outlet begins to increase. The reductant, disclosed as HC in Kibe""s preferred embodiment, does not according to Kibe otherwise contribute, by exothermic HC oxidation reactions, to heating of the catalyst mass or bed. The reductant is decreased to keep the catalyst within the operating temperature window.
Perhaps one of the more sophisticated approaches to using urea/ammonia system in a mobile application is disclosed in a series of patents which include U.S. Pat. No. 5,833,932 to Schmelz (Nov. 10, 1998); U.S. Pat. No. 5,785,937 to Neufert et al. (Jul. 28, 1998); U.S. Pat. No. 5,643,536 to Schmelz (Jul. 1, 1997); and U.S. Pat. No. 5,628,186 to Schmelz (May 13, 1997). While these patents discuss reducing reagents in a general sense, they are clearly limited to urea/ammonia reductants. According to this system, a catalytic converter having composition defined in the ""932 patent, has a reducing agent storage capacity per unit length that increases in the direction of gas flow. This allows for positioning of instrumentation along the length of the catalyst as disclosed in the ""536 patent to determine the quantity of ammonia stored in the catalyst. The catalyst is charged with the reducing agent such that transient emissions can be converted by the reducing agent stored in the catalytic converter. The ""186 patent, however, is directed as is the present invention, to a control system not limited to any specific catalyst. The ""186 patent recognizes, as does several prior art references discussed in this section, that i) sudden increases in load require a decrease in the reducing agent (and similarly sudden decreases in load require an increase in the reducing agent) and ii) the temperature (the ""186 patent also requires exhaust gas pressure) of the reducing catalyst affects its ability to store and release the reducing agent. The ""186 patent measures, from changes in gas pressure and catalyst temperature, the rate at which the reducing agent is being adsorbed or desorbed from the catalyst. It then calculates NOx emissions produced from the engine and sets a sub-stoichiometric ratio of reducing agent/NOx emissions at which the reducing agent is metered to the catalyst. The metering reducing agent rate is then adjusted upward or downward to equal the measured rate of reducing agent adsorption/desorption. A burner is provided to xe2x80x9cemptyxe2x80x9d the catalyst apparently to assure a sound reference value upon engine start for measurements and to guard against slip. Assuming the adsorption/desorption theory and measurement capability is xe2x80x9csoundxe2x80x9d, the system is sound although a large number of sensors and intensive calculations appear to be required.
Within the literature, a significant number of articles have been published investigating ammonia SCR NOx reducing systems and several articles have discussed control strategies to optimize the SCR NOx systems investigated. In SAE paper 921673, entitled xe2x80x9cDevelopment of an Ammonia/SCR NOx Reduction System for a Heavy Duty Natural Gas Enginexe2x80x9d by J. Walker and B. K. Speronello, (September 1992), various quantities of ammonia were injected at various engine speeds and loads to obtain optimum NOx conversions at steady state engine speeds and loads. The speeds and loads were mapped and stored in a look-up table (specific for each engine and each SCR catalyst) which was then accessed periodically to set an ammonia metering rate. This open loop, feed forward technique is conventionally used and produces good conversion ratios for steady state conditions.
SAE paper 970185, xe2x80x9cTransient Performance of a Urea deNOx Catalyst for Low Emissions Heavy-Duty Diesel Enginesxe2x80x9d by Dr. Cornelis Havenith and Ruud P. Verbeek (a co-inventor of the subject application) dated February, 1997 investigates ammonia metering adjustments made during transient emissions. A pulsed urea dosage device is disclosed which uses speed and load engine sensor data read into a control unit to pulse a quantity of ammonia in stoichiometric relationship to NOx emissions at steady state conditions. During step-urea, step-load and transient cycles, the stoichiometric relationship was decreased and a dynamic control strategy of injecting additional quantifies of urea after the transient or step or load was completed was adopted. A reduction in NOx emissions is reported although it is questionable whether the reduction was achieved because of the dynamic control strategy the additional reductant added during the transient or a combination thereof.
SAE paper 925022, xe2x80x9cCatalytic Reduction of NOx in Diesel Exhaustxe2x80x9d by S. Lepperhoff, S. Huthwohl and F. Pischinger, March, 1992 is an early article that looked at step load changes to evaluate transient systems. The article recognizes that when the load on the engine changed at constant rpm, the NOx emissions increase, the temperature increases and the total exhaust flow increases. Response of the catalyst to step changes in the engine operating conditions are referred to as step load tests. Ammonia slip occurred when engine load increased and the article concludes the slip is correlated to the ammonia stored in the catalyst. It was suggested that a control program or control system would have to consider the NOx emissions of the engine, the catalyst temperature and the ammonia stored within the catalyst to avoid ammonia slip.
SAE paper 952493, xe2x80x9cAn Urea Lean NOx Catalyst System for Light Duty Diesel Vehiclesxe2x80x9d by H. Luders, R. Backes, G. Huthwohl, D. A. Ketcher, R. W. Horrocks, R. G. Hurley, and R. H. Hammerle, October, 1995 concludes that an ammonia SCR system can control NOx diesel emissions. The control strategy used in the study was similar to that disclosed in the Gladden and Lane patents above i.e., a microprocessor mapped engine out NOx emissions and catalytic converter temperature. Engine out NOx was derived from engine speed and torque. Space velocity (intake air mass flow) and catalyst temperature were then used with NOx out data to set a maximum NOx reduction rate. Transient operation was numerically modeled from steady state conditions. Ammonia storage and thermal inertia was noted as factors affecting the conversion but the control system discussed had no special provisions, other than numerical modeling.
SAE paper 930363, xe2x80x9cOff-Highway Exhaust Gas After-Treatment: Combining Urea-SCR, Oxidation Catalysis and Trapsxe2x80x9d by H. T. Hug, A. Mayer and A. Hartenstein, March, 1993, describes stoichiometric injection of ammonia, without lag, based on engine mapped conditions. Catalyst porosity is stated to be important with respect to transient emissions. An injection nozzle for metering is disclosed.
An article entitled xe2x80x9cNOxxe2x80x94Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reductionxe2x80x9d by M. Koebel, M. Elsener and T. Marti, published in Combustion Science and Technology, Vol. 121, pp. 85-102, 1996 describe experiments conducted xe2x80x9cat abrupt load changesxe2x80x9d. An abrupt reduction in load did not cause ammonia slip but an abrupt increase in load did cause ammonia slip. The article observes that the catalyst is saturated with adsorbed ammonia at lower temperatures; that increased load significantly increases NOx emissions; that increased load increases, slowly, the temperature of the catalyst. Ammonia slip occurring at the onset of the abrupt load change because of excessive ammonia present when the desorption of the ammonia is increased while the bulk at of the catalyst bed is too cool to effectively react the desorbed ammonia with the higher level of NOx. This observation has been noted in several of the prior art references discussed above. The recommendation is to retard the addition of ammonia in relation to the load increase.
In general collective summary of the prior art references discussed above, it is known that ammonia SCR systems can be used effectively to control the emissions produced by vehicles powered by diesel engines; that the reducing catalysts adsorbs and stores ammonia at low temperatures and desorbs the stored ammonia at higher exhaust gas temperatures; that steady state NOx emissions, determined from mapped speed and load engine conditions, can be readily controlled by metering ammonia in stoichiometric relationship to the NOx emissions; that it is possible to pump urea, react urea to produce ammonia and precisely control the rate of ammonia rejection to the exhaust gases by controlling pulsed injections of ammonia; and that transient emissions cause transient increases in NOx concentrations with attendant exhaust gas temperature increases requiring a reduction in the ammonia metering rate to balance the increased ammonia present attributed to desorption resulting from the temperature increase. Noticeably absent, from any of the mobile applications discussed, is a simple control system capable of quickly and effectively adjusting the metering rate during transient emissions as well metering the reductant during steady state operating conditions.
In this regard and as noted above, industrial processes, which do not have the sudden transient emission changes of a vehicular application, can utilize NOx sensors in a closed loop controlled through set-point controllers. There are no commercially available NOx sensors which have the response time needed for vehicular applications. Thus any SCR control system for mobile applications will necessarily be open loop.
Accordingly, it is a principal object of the present invention to provide a control for an NOx SCR mobile emission reduction system which is able to control the system to reduce transient as well as steady state NOx emissions without reductant slip.
This object along with other features of the invention is achieved in a method for reducing NOx emissions produced in mobile diesel applications by an external reductant supplied to an SCR system comprising the steps of a) sensing one or more engine operating parameters to predict a concentration of NOx emissions indicative of the actual quantity of NOx emissions produced by the engine; b) when the actual concentration of NOx emissions changes and the temperature of said reducing catalyst is within a set range, varying the actual concentration of NOx emissions by a time constant to produce a calculated concentration of NOx emissions different than the actual concentration of NOx emissions; and, c) metering the external reductant to the reducing catalyst in said SCR system at a rate sufficient to cause the reducing catalyst to reduce said calculated concentration of NOx emissions whereby metering of the reductant accounts for the effects on said SCR system attributed to transient NOx emissions. More particularly, the NOx constant acts to decrease the actual concentration of NOx emissions when the NOx emissions increase to avoid reductant slip and increase the actual NOx emissions when the NOx emissions decrease to utilize the catalyst reductant storage abilities.
In accordance with an important feature of the invention, the NOx time constant is a function of the catalyst temperature within a set temperature range as that catalyst temperature relates to the capacity of any given catalyst to store reductant at that temperature. Generally, the storage ability of the catalyst decreases as the catalyst temperature increases within the catalyst temperature range whereby the reductant is metered, during and following an NOx transient, on the basis of the ability of any specific catalyst used in the SCR system to store the reductant thus minimizing the likelihood of reductant slip.
It is a distinct feature of the invention that the ability of any given catalyst to store reductant is expressed as the relative time it takes for any given catalyst to store reductant at any given catalyst temperature to generate a varying time constant that can be accessed through a conventional look up table storing time constant-catalyst temperature relationships. The time constant is utilized to account for the lag in the catalyst response to the NOx transient by modifying the NOx emission concentration in any number of ways, such as by determining a moving average of NOx emissions over varying time periods, each time period correlated to a time constant in the look-up table for a then current catalyst temperature, so that reductant dosage is determined without having to sense numerous parameters and perform numerous calculations to periodically determine current storage capacity of the catalyst for setting the reductant metering rate.
However, it is a distinct feature of the invention to provide a filter to account for the lag in the catalyst system attributed to transient NOx emission by filtering the actual NOx emissions (increasing or decreasing) to NOx concentrations which do not exceed the catalyst""s ability to store reductant at its current temperature in a responsive and robust control. In accordance with this feature of the invention, the filter uses the capacity of the catalyst to store reductant at the lower temperatures of the catalyst temperature range while also providing, when the reductant is aqueous urea, improved urea hydrolysis by the provision of two first order filters in series represented in the continuous time domain by the transfer function:       H    ⁢          (      s      )        =            1                                    τ            1                    ·          s                +        1              ·          1                                    τ            2                    ·          s                +        1            
and 
xcfx841=xcfx842=ƒ(Cat) 
When filtering the actual NOx emission concentration, the variable NOx time constant, xcfx84NOx, is determined from the look-up table noted above as a function of the catalyst temperature. In accordance with the broader scope of the invention, the second order filter is effective to introduce a lag for any temperature dependent relationship of the catalyst, including but not limited to those that are only straight line or constant approximations of the catalyst""s ability to store reductant at certain temperatures within a temperature range of the catalyst.
In accordance with another distinct aspect of the invention, the system employs a second order filter, as represented in the continuous form designated above, to account for the changing heat fronts moving through the catalyst bed which are attributed to NOx transient emissions and produces a functional catalyst temperature which is a more accurate temperature than that achieved by sensing pre or post or mid-bed catalyst temperatures. In accordance with this aspect of the invention, (which is not limited in application to control systems which factor NOx concentrations but can be applied to any mobile system which measures or senses catalyst temperature for any reason), the catalyst time constant xcfx84Cat is a function of the space velocity of the exhaust gases through the catalyst.
In accordance with this distinct feature of the invention, a method for determining the functional temperature of a catalyst in an exhaust system of a vehicle includes the steps of i) determining, by sensing or calculating, the temperature of the exhaust gases and the space velocity of the exhaust gases through the catalyst and ii) filtering the exhaust gas temperature by a catalyst filter to generate the functional temperature of the catalyst. Significantly, the catalyst filter implements a time constant determined as a function of changing space velocity to filter the exhaust gas temperature and is implemented in the continuous time domain by a second order filter as set forth above.
In accordance with a still further feature of the invention, the transfer function, H(s), for the NOx and catalyst filters of the present invention can be easily implemented in any number of discrete forms into the vehicle""s existing microprocessor because any of the conventional difference equations implementing the transfer function in discrete form are not memory intensive.
In accordance with yet another aspect of the invention, the two first order filters forming the functional catalyst temperature are split, upon engine shut-down, from a series relationship into two individual parallel operating first order filters with ambient temperature fed as filter input to the temperature of the catalyst so that after short engine stop/start periods, the cooled down temperature of the catalyst is used in the second catalyst filter to prevent reductant slip after engine restart. In accordance with this aspect of the invention, the cool down time constant, xcfx84Cool, is determined as a function of time elapsed from vehicle shut down or parameters that represent difference in temperature. By arranging both filters in parallel so that each receives the same information, the second parallel filter is prevented from freezing or drifting when the filters are switched back to series relationship upon restart of the vehicle.
In accordance with a specific feature of the invention, the external reductant is ammonia and the storage capacity of the reducing catalyst which is used to set the time constant xcfx84NOx for any given catalyst is a function of a) the surface area of the catalyst over which the exhaust gases flow, b) the number and strength of adsorption/absorption sites on the surface area and c) the ability of the catalyst washcoat to store NOx at any given temperature within the set temperature range whereby a control method not only uses a well known reductant to optimize the performance of any given catalyst, but also provides a method to optimally size a reducing catalyst for any given engine/vehicle combination to meet regulated drive cycle NOx emission requirements.
In accordance with a still further feature of the invention, the external reductant is metered at a normalized stoichiometric ratio of reductant to NOx emission established for the current functional catalyst temperature (as determined by the catalyst NOx constant) whereby each essential step of the method, i.e., the NOx emissions, the catalyst functional temperature and the NSR ratio, have all been adjusted to account for the inevitable effects on the SCR system resulting from engine operating conditions producing NOx transient emissions which otherwise adversely affects the operation of the SCR system. Again, the current state of the catalyst does not have to be sensed nor intensive calculations run based on sensed catalyst state to set the reductant rate.
Still another specific and inclusive feature of the invention is to provide a method for metering an external reductant to a reducing catalyst in an SCR system applied to a vehicle powered by an internal combustion engine which includes the steps of
a) sensing operating conditions of the vehicle and engine to generate, by calculation and/or measurement, signals indicative of the actual quantity of NOx emissions emitted by the engine, the temperature of the exhaust gas and the space velocity of the exhaust gas;
b) filtering, when the temperature of the catalyst is within a set temperature range, the actual NOx emission signal by an NOx time constant to produce a calculated NOx signal different than the actual NOx signal when the NOx signal is changing;
c) filtering the exhaust gas temperature signal by a catalyst time constant to produce a functional catalyst temperature signal different than the exhaust gas temperature when the space velocity signal changes;
d) factoring the functional catalyst temperature signal and the space velocity signal to generate a NSR signal indicative of a normalized stoichiometric ratio of reductant to NOx emissions; and,
e) metering the reductant to the reducing catalyst by factoring the calculated NOx signal by the NSR signal to produce a metering signal controlling a metering device for the external reductant.
It is a general object of the invention to provide a nitrogen based SCR control system for NOx emissions produced by diesel powered vehicles.
It is another general object of the invention to provide an external reductant SCR control system for mobile IC engine applications which minimizes reductant slip while utilizing the ability of the SCR catalyst to store reductant.
It is an object of the invention to provide a control system for mobile NOx SCR systems having any one or any combination of the following characterizing features:
a) Ability to control reduction of transient as well as steady state NOx emissions;
b) Ability to prevent reductant slip during NOx transient emissions;
c) Simple to implement in programmable routines not subject to extensive memory requirements stored in the ECU;
d) Robust, stable and not subject to significant drift over time;
e) Able to account for thermal aging of catalyst;
f) Easily implemented in OBD diagnostic systems;
g) Inexpensive because it requires no additional parts or components other than what is currently used in state-of-art systems;
h) Not limited to any specific driving cycle or test cycle; and
i) Insensitive to arbitrary changes to temperature and/or load and/or NOx emissions.
Another distinct but related object of the invention to provide a method for determining the functional catalyst temperature of SCR catalysts for use in any control type system resulting from changes attributed to NOx transient emissions notwithstanding what methodology is used to establish the catalyst temperature at steady state conditions.
Still another stand alone but related object of the invention is to provide a method for ascertaining the start-up temperature of any catalyst in any emission system.
Still yet another object of the invention is to provide a control system for a mobile IC engine SCR application which is able to account for changes to the SCR system attributed to NOx transient emissions notwithstanding the fact that such control systems may employ NOx and/or reductant sensors assuming commercially acceptable, time responsive sensors are developed for mobile engine applications.
Still another object of the invention is to provide an SCR control system for mobile IC applications using an external reductant which can function with any design or type of reducing catalyst used in the SCR system.
Another object of the invention is the provision of an SCR control system for mobile IC applications using an external reductant which provides a basis for optimizing the selection of any specific reducing catalyst for any given engine/vehicle combination.
Yet another object of the invention is to provide an SCR control system for an external reductant which determines and uses the storage/release capacity of reductant and NOx emissions for any given SCR catalyst to control reductant metering in a manner that accounts for the capability of that specific catalyst to reduce NOx emissions as a result of NOx transient emissions produced by the engine.
A still further object of the invention is the provision of a control system for an external reductant applied to a mobile IC engine having an SCR system in which reductant usage is minimized while maintaining high NOx conversion.
Another object of the invention is to provide a control system for an external reductant SCR system which emulates the lag of the catalyst following NOx transient emissions by use two simple first order filters in series having time constants determined as a function of temperature.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the Detailed Description of the Invention set forth below taken in conjunction with the drawings.